"Comets are like cats, they have tails and they do what they want" once said Canadian astronomer David H Levy, who co-discovered Comet Shoemaker-Levy 9.

Of course, comets are not only like cats, they are like buses; you can wait ages for one, then a whole lot turn up all at once.

Earlier this year we saw Comet PanSTARRS and the fainter Comet Lemmon.

At the moment amateur astronomers are focused on a "comet convoy" in the morning sky. There's C/2013 R1 Lovejoy, discovered by Australian amateur astronomer Terry Lovejoy; the old reliable comet 2P/Enke, which orbits the Sun every three years; and C/2012 X1 LINEAR, which just brightened an astonishing 250 times.

You may be surprised by the number of comets just mentioned, but every year a handful or so of comets become "bright" by astronomers' standards. That is, they become bright enough to see in modest size scopes accessible to amateurs. Every couple of years we get a comet visible to the unaided eye, usually as fuzzy dot you can see only out in the bush.

And roughly every five years on average, we get a really bright comet.

In 2007, we saw the astonishing C/2006 P1 McNaught, a bright dusty comet with a rooster tail named after Dr Rob McNaught, the prolific Australian astronomer who discovered it, then in 2011 the 'Christmas Comet', C/2011 W3 Lovejoy graced our skies.

Now there is rising anticipation as we watch the development of comet C/2012 S1 ISON, dubbed the "Comet of the Century" (rather unfortunate, as the last comet called "Comet of the Century" was underperforming C/1973 E1 Kohoutek).

So far, despite a significant outburst on 14 November, ISON has been much less brighter than predicted. It may yet produce a beautiful, if not spectacular display after it passes the Sun, but, sadly, it is likely that it will not get bright enough to see without binoculars from Australia.

Comet of the Century?

Coming soon: Taken on 28 October this image by UK-based astrophotographer Damian Peach shows the glowing green nucleus and fainter tail of ISON as it travels towards the Sun
(Source: Damian Peach)

For a comet to be really bright, it must come reasonably close to the Sun and Earth, it must have a good dust to gas ratio, and it should be reasonably large.

Comets are fairly fragile masses of dust and ices (including not only water ice, but frozen carbon dioxide, carbon monoxide and a range of other exotic gasses). Comets are also much less dense than pure water ice, suggesting that there is a lot of empty space and that comets are about as solid as an ice cream sundae.

Most comets are at their brightest after perihelion, their closest approach to the sun, as the heat of the sun vaporises the ices in the comet blasting out masses of dust. Because it takes a while for heat to penetrate the comet's surface, even close to the Sun, the comet will generally pump out more dust and gas after perihelion than before.

Comet ISON certainly fulfills the first criterion, coming almost within a solar diameter of the Sun's surface. This is extremely close, and the comet is classified as a sungrazer. However, this is the source of one of the anxieties about this comet — sungrazers rarely survive.

Should it survive its baptism of fire, comet ISON comes reasonably close to Earth, about half the distance from Earth to the Sun. But this is quite some time after the comet is at its brightest.

A dusty comet is usually brighter than a gas dominated comet. The amazing "rooster tail" of comet McNaught was dominated by dust, as was the thin searchlight of Lovejoy. A gas-dominated comet tends to have a very dim tail, as with comet Lemmon earlier this year.

Currently astronomers are poring over spectra of comet ISON trying to determine which kind it is. There is a lot of gas, but a lot of dust as well. Time will tell. Perhaps we will have a comet like that of Comet West, with a bright dust tail and a bright blue tail of ionised gas.

The bigger the comet, the more dust can be released, and the brighter the tail. Comet ISON is reasonably big. It is hard to be certain, but measurements from the Spitzer and Hubble Space Telescopes put an upper limit of five kilometres for its diameter, and the figure of two kilometres has been bandied about. That still makes it bigger than comet Lovejoy, which as a fair bit under a kilometre in diameter when it rounded the Sun, leaving us with a pale, but spectacular tail.

So why was comet ISON dubbed the 'Comet of the Century'?

When C/2012 S1 was discovered last year it was far beyond Jupiter's orbit, but relatively bright. This brightness, along with its sungrazing orbit, suggested it would become very bright, possibly as bright as the full moon. However, the brightness equations were for comets that had travelled into the inner solar system previously. It soon turned out that ISON is a fresh comet from the Oort Cloud out on the very edge of the solar system, and this is its first journey close to the sun.

Fresh comets brighten rapidly early on, but then brighten more slowly close to the Sun. This is possibly due to a "hoar frost" of volatile gases blowing off surface dust, until they are depleted. Since then comet ISON has brightened steadily — although more slowly than hoped for — and has a beautiful tail (if you have a telescope to see it).

ISON in the Southern Hemisphere

When McNaught and Lovejoy were at their best they were almost exclusively visible from the Southern Hemisphere. But this time, people living in the Northern Hemisphere will get the best view of ISON — if it survives its trip around the sun on 28 November.

Up until then, it can be seen low above the eastern horizon in the southern hemisphere as it travels towards the sun. But it will get increasingly harder to see. As ISON moves closer to the Sun, it not only gets closer to the horizon, but also as it brightens, the sky is brightening as well. So what we see all comes down to the race between how fast the comet brightens, and how fast the sky brightens as the comet draws closer to the dawn.

Your view of the comet depends upon the latitude where you live.

The best views will be in the Far North, at or above the latitude of Cairns, with Darwin having the comet highest above the horizon of all the major cities.

At the beginning of the month, ISON was bright enough to see under dark skies with strong (10 x 50 binoculars) in the constellation of Leo.

As it has moved towards the sun over the last three weeks it didn't brightened as predicted.

Then on 14 November there was a strong outburst and the comet became over 10 times brighter in a short time. At that time, ISON was been reported to be visible with the unaided eye as a fuzzy dot in Europe, but visual observations in Australia have been less successful.

By 18 - 19 November the comet should have brightened rapidly and be easy to find less than a finger-width from the star Spica — the brightest star in the constellation of Virgo — but although it has brightened, it is around magnitude 5 and hard to see low in the twilight sky. People in Brisbane, Townsville and further north reported seeing it in binoculars.

This week the comet begins the final leg of its long journey to the Sun.

The comet is sufficiently low now that you need to start looking toward the eastern horizon an hour before local sunrise — a time known as nautical twilight — or even earlier as it sinks into the brightening dawn sky.

If you live north of Brisbane you'll need binoculars to locate it. If you live north of Perth or Sydney you can also still just see a fuzzy blob with binoculars, but by 23 November you'll need to use a good telescope and have a clear horizon to see it. Unfortunately, if you live further south, the comet has now slipped out of view.

On 28 November, if ISON rounds the Sun, it may be bright enough to be theoretically visible in daylight, but as it will be only a finger width from the Sun, do not attempt to observe it. Unless you are a very experienced observer with specialised equipment you risk severe eye damage or blindness from the Sun.

This time is crucial for comet ISON's development, if the comet disintegrates completely then its journey is over. If it survives, or breaks into two or three big pieces, then there should be a spectacular tail, somewhere between that splendor of McNaught's tail and the pale searchlight of Lovejoy.

If you live at the latitudes of Cairns or above you may be lucky enough to see the comet tail poking up above the horizon after November. Hopefully, the rest of us will be able to vicariously enjoy comet ISON via images from the northern hemisphere in December as it returns to the depths of space.

Bright beauties

Admire the celestial beauty of some of the greatest — and not so great — comets of our time. If it does survive its journey around the Sun, it is hoped ISON will look similar to Comet Lovejoy. But as we said earlier: comets are like cats.

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How are comets named?

C/2012 S1 ISON, what kind of comet name is that? You may be more familiar with comets named after their discoverers, like comet Halley (he didn't actually discover the comet that bears hs name, but was the first to work out that several comet apparitions were one and the same comet, and predicted its return). But in this era of robotic telescopes like LINEAR making multiple discoveries having 120 or so comets named LINEAR is quite confusing. Or when you say "comet McNaught" , which of the 82 comets discovered by Rob McNaught, one of the world's most prolific comet observers, do you mean?

So now there is a systematic naming system, the first letter either "C/" for comet that will not return, or a "P/" for a periodic comet (one that returns several times). Then comes the year the comet was discovered, then the discoverer (either one or more people, or a robot telescope, ISON was discovered by the International Scientific Optical Network), then a letter/number code showing what part of the year the comet was discovered and in which order they were discovered. S1 means that the comet was the first comet discovered in the second half of September.

About the author:Dr Ian Musgrave is an avid amateur astronomer. He writes the weekly sky updates for ABC Science and is science adviser to iTelescope.net. When not staring at the sky he is an equally enthusiastic molecular pharmacologist at the University of Adelaide, Australia. You can follow him on Astroblog for daily posts about astronomy, biology and life, the Universe and everything.

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